1. Field of the Invention
This invention relates to gallium arsenide (GaAs) high electron mobility transistors (HEMTs), in particular, techniques to increase the resistance of HEMTs with respect to neutron radiation.
2. Description of the Prior Art
GaAs HEMTs constitute an increasingly important class of semiconductor. A standard HEMT (also called a MODFET Modulation-Doped Field Effect Transistors) offers speed and power advantages over GaAs MESFET, particularly at cryogenic temperatures.
A typical HEMT device is shown in FIG. 1. A GaAs buffer layer 2 is grown by molecular beam epitaxy (MBE) on the semi-insulating GaAs substrate 4. This is followed by 550 .ANG. of Al.sub.0.25 Ga.sub.0.75 As, the first 30 .ANG. being an undoped spacer layer 8 and the next 520 .ANG. layer 10 doped with Si at a concentration of 1.times.10.sup.18 cm.sup.-3. Next, a highly doped 300 .ANG. GaAs layer 12 is grown on top of the Al.sub.0.25 Ga.sub.0.75 As layer 10 to facilitate making ohmic contacts with the source, drain and gate. The concentration of Si in the second doped layer is about 2.times.10.sup.18 cm.sup.-3.The contacts for source 14 and drain 16 are standard alloyed AuGe/Ni/Au and the contact for gate 18 is Ti/Pd/Au.
However, the use of HEMTs in space systems has been limited by the susceptibility of HEMTs to neutron radiation. In particular, HEMTs with AlGaAs/GaAs heterojunctions are susceptible to threshold voltage changes induced by neutron radiation.